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Article

Surface Engineering of Multi-Walled Carbon Nanotubes via Ion-Beam Doping: Pyridinic and Pyrrolic Nitrogen Defect Formation

by
Petr Korusenko
1,2,*,
Ksenia Kharisova
3,
Egor Knyazev
2,4,
Oleg Levin
3,
Alexander Vinogradov
1 and
Elena Alekseeva
3
1
Department of Solid State Electronics, V.A. Fock Institute of Physics, St. Petersburg State University, 7/9 Universitetskaya nab., 199034 St. Petersburg, Russia
2
Department of Physics, Omsk State Technical University, 644050 Omsk, Russia
3
Electrochemistry Department, St. Petersburg State University, 7/9 Universitetskaya nab., 199034 St. Petersburg, Russia
4
Laboratory of Physics of Nanomaterials for Chemical Current Sources, Omsk Scientific Centre Siberian Brunch of Russian Academy of Science, 644013 Omsk, Russia
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(19), 11057; https://doi.org/10.3390/app131911057
Submission received: 8 September 2023 / Revised: 25 September 2023 / Accepted: 5 October 2023 / Published: 8 October 2023
(This article belongs to the Special Issue Practical Application of Functionalized Carbon-Based Nanomaterials - Volume Ⅱ)
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Abstract

:
In this study, we present an innovative ion-beam doping technique for the controlled modification of the near-surface region of multi-walled carbon nanotubes (MWCNTs) aimed at creating pyridinic and pyrrolic nitrogen defects in their walls. This method involves the irradiation of MWCNTs with nitrogen ions using a high-dose ion implanter, resulting in the incorporation of nitrogen atoms into the nanotube structure. The structural and chemical changes induced by the ion-beam treatment were thoroughly characterized. Scanning electron microscopy (SEM) analysis revealed subtle changes in nanotube morphology, while X-ray diffraction (XRD) measurements exhibited altered peak intensities and a shift in the (002) reflection peak, indicating structural modifications, which correlates with transmission electron microscopy (TEM) data. X-ray photoelectron spectroscopy (XPS) analysis confirmed the successful embedding of nitrogen, mainly in pyridinic and pyrrolic configurations, as evidenced by the presence of corresponding lines in the N1s spectrum. Our findings demonstrate the feasibility of precisely engineering nitrogen defects in MWCNTs using the ion-beam doping technique. This approach is expected to be promising for the use of carbon nanotubes surface-functionalized with nitrogen atoms in the development of new devices for electronics, electrochemistry, catalysis, etc.

1. Introduction

The extraordinary combination of distinctive physical and chemical properties of carbon nanotubes (CNTs)—including high thermal and electrical conductivity, large surface area per unit mass, robust mechanical properties, a wide range of electronic properties, and promising potential for practical applications—has clearly captured the imagination of the research community [1,2,3,4]. Specifically, CNTs promise to emerge as fundamental components for advanced high-strength composites [5,6], sensitive components in gas sensors [7], and key elements in lithium-ion batteries [8]. At the same time, the need for effective materials to facilitate the cathodic oxygen reduction reaction (ORR), cold electron emitters, and chemical capacitor electrodes has increased the demand for nitrogen-doped carbon nanotubes (N-CNTs) [9,10,11,12,13]. This requirement is supported by the incorporation of nitrogen in various chemical configurations, intricately integrated into the graphene walls of the nanotubes. This procedure, in turn, exerts a profound influence on the physicochemical properties of these nanotubes. Specifically, this influence induces an increase in the concentration of n-type charge carriers, an increase in the surface energy, and a decrease in the work function. These configurations are based on three different nitrogen defect arrangements in graphene, illustrated in Figure 1: the pyridinic, pyrrolic, and graphitic (or quaternary) forms [14,15,16].
In the pyridinic configuration, two nitrogen electrons actively participate in σ bonds with adjacent carbon atoms, one electron contributes to the π system of the CNT, and the remaining two electrons form a lone pair localized near the nitrogen atom. On the other hand, within the pyrrolic configuration, three nitrogen electrons form σ bonds with carbon atoms and the two remaining electrons participate in π-bonding [10,13,14]. In addition, graphitic (quaternary) nitrogen refers to a nitrogen atom that replaces carbon atoms in benzene rings without creating vacancies [10,13,14]. In this context, the presence of one extra electron on the nitrogen atom makes an additional contribution to the π subsystem of CNTs. Such an increase in electron density can have a significant impact on the density of electronic states near the Fermi level, thereby playing a crucial role in the desired change and improvement in the physicochemical properties of nanotubes [17,18,19].
The inclusion of nitrogen into CNTs, known as nitrogen doping, can be achieved through two primary approaches: during CNT synthesis (in situ) or post-treatment (ex situ) methods [18,19,20,21,22]. In situ doping involves directly incorporating nitrogen heteroatoms into the carbon matrix during the synthesis process and is commonly employed to produce nitrogen-doped CNTs (N-CNTs). Among the various in situ methods, chemical vapor deposition (CVD) and pyrolysis are widely used due to their ability to effectively incorporate nitrogen atoms into the carbon nanotubes [23,24]. These methods use a wide range of carbon sources (acetylene, ethylene, benzene, methane, toluene, etc.) and nitrogen sources (acetonitrile, ethylenediamine, dimethylformamide, benzylamine, etc.) [25,26,27,28,29,30,31,32]. Despite their advantages, including the high product yield, relatively low production costs, and flexibility to manipulate synthesis parameters (such as pyrolysis temperature, gas flow rate, and synthesis duration), these methods suffer from limitations. Notably, their lack of precise reproducibility arises from the significant influence of various factors (precursor concentration, gas flow rate, catalyst distribution, substrate type, temperature gradient in the CVD reactor, residual impurities, etc.) on the resulting morphology and structure of N-CNTs, as well as the nitrogen content and its distribution in nanotubes.
An alternative approach, post-treatment, is favored due to the availability of commercial undoped CNTs with specific physicochemical or physico-mechanical traits. In post-treatment, nitrogen doping is typically achieved using gaseous ammonia or by pyrolyzing nitrogen precursors such as acetonitrile, pyridine, melamine, and polyvinylpyridine [33,34,35,36,37,38]. However, post-treatment necessitates high temperatures (800–1200 °C) and often employs toxic nitrogen precursors (acetonitrile, NH3, pyridine), limiting its practical utility. Additionally, the multi-step nature of the post-treatment procedure contributes to the elevated production costs of N-CNTs. To address these challenges, ion and plasma treatment methods are promising, offering precise control over nitrogen doping parameters [39,40,41,42,43,44,45]. Studies have demonstrated that plasma treatment, influenced by parameters like plasma power, pressure, and exposure time, can lead to N-CNTs with varying nitrogen content (up to 8–20 at.%) embedded into their wall structure [43,44]. Notably, the nitrogen inclusions in the side walls of N-CNTs predominantly assume pyridinic and graphitic configurations, while the tips of nanotubes exhibit a pyrrolic configuration during plasma treatment [44]. In comparison, treatment with N+ ions, akin to plasma but distinct in its capability to modify both the outer surface layers and inner bulk layers of nanotubes by adjusting the average ion beam energy, offers a compelling avenue [44,46,47,48]. This method involves implanting nitrogen within both wall structures (graphitic/quaternary and pyridinic forms) and tip regions (primarily pyrrolic and graphitic/quaternary forms), resembling the outcomes of plasma treatment [44,48]. Despite recent advancements in nitrogen doping of carbon nanomaterials, challenges persist in achieving N-CNTs with a predetermined nitrogen content and specific configurations in targeted locations on nanotube walls, necessitating innovative approaches.
Earlier, our team carried out ion doping with nitrogen atoms of CNTs to improve their electrochemical activity [49]. At the same time, irradiation was carried out on pressed CNT powder, as well as prepared electrodes, and a comparison was made with nitrogen-doped CNTs at the stage of CVD synthesis. Such an energy impact led to local destruction of the electrodes and to a decrease in the stability of such materials during cyclic charge–discharge processes.
The primary goal of this study is to develop an ion-beam doping technique for nitrogen incorporation in multi-walled CNTs (MWCNTs). This approach aims to produce N-MWCNT powder (about 100 mg per irradiation cycle) with a predominant presence of pyridinic and pyrrolic nitrogen forms, aligning with their potential applications in electrocatalysis and the oxygen reduction reaction (ORR). The proposed methodology involves depositing a 650–700 nm thick layer of MWCNTs on a titanium substrate using the spray method, followed by irradiation with N+ ions using a high-dose ion implanter. A comprehensive set of experimental techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) is used to elucidate the resulting morphology, structure, and chemical states of carbon and nitrogen atoms.

2. Materials and Methods

2.1. Sample Preparation

Initial undoped carbon nanotubes were sourced from the “MWCNT-1” brand (manufactured by the G.K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences (SB RAS)), obtained via chemical vapor deposition (CVD) [50].
The sample preparation procedure is schematically depicted in Figure 2. In the initial step, a suspension was prepared by mixing MWCNT powder with ethanol (0.005 g of MWCNT powder per 15 mL of ethanol), followed by ultrasonic treatment (Jeken Ultrasonic Cleaner model PS-06A, Dongguan City, Guangdong Province, China) for 5 h. Subsequently, the prepared suspension was aerosol-sprayed onto a preheated titanium substrate using an airbrush, with the substrate temperature maintained at 65–70 °C for around 5–7 min (Figure 3a). This process yielded a layer of MWCNTs approximately 650–700 nm in thickness (Figure 3b). Ultimately, the formed MWCNT layer was irradiated by a continuous ion beam using an accelerator located at the Omsk Scientific Center, SB RAS (Figure 3c). A schematic representation of the ion source in this accelerator is shown in Figure 3d. The principle of operation of the source is based on the Penning discharge and is as follows: after creating a preliminary vacuum of about 10−4 Torr in the accelerator chamber, the working gas enters the ion source for ionization in crossed magnetic and electric fields. The ions of the obtained low-temperature plasma, when an accelerating voltage is applied, bombard the surface of the samples placed on the sample table. It should be noted that such a design of the ion source makes it possible to change the beam cross-section in accordance with the dimensions of the substrate; thus, it is possible to irradiate the surfaces of materials with an area of up to 8000 cm2. In our case, the pressure of residual gases in the accelerator chamber after pumping out was 3 × 10−5 Torr. After the working gas (dry N2) was admitted, the pressure was 3 × 10−4 Torr. This irradiation involved N+ ions with an average energy of 20 keV and the beam current was about 100 mA. The choice of average ion energy considered the range of nitrogen ions within a layer of specific thickness, as determined using the SRIM-2013 software package (refer to Figure 3e) [51]. Experimentally, the beam fluence at this stage was selected based on visual inspection to minimize surface damage. An optimal beam fluence of 1 × 1017 ion/cm2 (which corresponds to a current density of ~50 μA/cm2) was determined to yield satisfactory results; thus, this modification mode was adopted for the initial experiments.

2.2. Sample Characterization

Before measurements, N-MWCNTs were scraped off the titanium substrate with a surgical scalpel to form a powdery mass. In the case of SEM, modified nanotubes were deposited on a polycrystalline silicon wafer. For TEM measurements, a colloidal solution was prepared by adding the test sample (CNT powder) to ethyl alcohol, followed by keeping it in an ultrasonic bath until a homogeneous dispersion appeared. After this, the colloidal solution was transferred using a microdispenser to a special ultra-thin carbon membrane on a copper frame for subsequent TEM research.

2.2.1. SEM

Information on the morphology of the samples was obtained using a Carl Zeiss Merlin (Carl Zeiss Microscopy, Jena, Germany) at the research park of St. Petersburg State University (Interdisciplinary Center for Nanotechnology, St. Petersburg, Russia). SEM images were recorded at a voltage of 10 kV and magnifications of 100,000 and 300,000 times.

2.2.2. TEM

The structure of the samples was analyzed via Carl Zeiss Libra 200 FE (Carl Zeiss Microscopy, Jena, Germany) equipped with a highly efficient field emission emitter and an OMEGA energy filter in the research park of St. Petersburg State University (Interdisciplinary Center for Nanotechnology, Russia). TEM images were registered at an accelerating voltage of 200 kV with various magnifications from 20 kX to 400 kX.

2.2.3. XRD

X-ray diffraction analysis was carried out to determine the crystalline structure of CNTs before and after modification. XRD measurements were made using Cu-Kα radiation at an accelerating voltage of 30 kV via a Bruker D2 Phaser X-ray diffractometer (Bruker Corporation, Billerica, MA, USA). The X-ray patterns were recorded in the range 2θ = 10–80° with a step of 0.02° at the research park of St. Petersburg State University (Centre for X-ray Diffraction Studies, St. Petersburg, Russia).

2.2.4. XPS

Photoemission spectra (survey and C1s, N1s lines) were measured using an ESCALAB 250 Xi laboratory spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) in the constant-energy mode of the analyzer with pass energies of 50 and 10 eV. Monochromatized AlKα radiation (1487 eV) was used in the measurements. Calibration of the binding energy scale was carried out using pure gold foil, for which the Au 4f7/2 peak and the Fermi level were determined. Detailed analysis of the C1s and N1s PE spectra was conducted through peak fitting employing Casa XPS 2.3.16 software [52]. All XPS measurements were performed at the research park of St. Petersburg State University (Centre for Physical Methods of Surface Investigation, St. Petersburg, Russia).

3. Results

The experimental studies carried out showed that for one cycle of nitrogen ion irradiation of a layer of deposited MWCNTs with a thickness of about 650–700 nm on a round titanium disk with an area of 30 cm2, approximately 1 mg of N-MWCNT powder is obtained. At the same time, the dimensions of the accelerator chamber and the design features of the ion source, due to the possibility of supplying a potential to the sample, make it possible to irradiate round samples with an area of up to 8000 cm2, which theoretically corresponds to about 100 mg of N-MWCNT powder per irradiation cycle. Next, we characterize nitrogen-modified carbon nanotubes in detail in comparison with the pristine MWCNTs using SEM, TEM, XRD, and XPS techniques.
The SEM images in Figure 4 show that the untreated MWCNTs are compared with those subjected to irradiation with a nitrogen ion beam. Examination of these SEM images did not disclose substantial alterations in the morphology of individual nanotubes prior to and following exposure to N+ ions. However, for the N-MWCNTs sample, a slight increase in the average outer diameter (d) of individual nanotubes was observed from approximately 16–18 nm to about 22–25 nm. The observed change in parameter d subsequent to irradiation can be attributed to the partial sputtering of outer nanotube layers, along with the fusion of individual nanotubes at specific locations. Previously, we observed a similar effect when MWCNTs were irradiated with argon ions [13,14,27,47]. This fusion may be associated with the creation of point and extended structural defects as a result of carbon atom removal from nanotube walls. Such removal elevates the free energy of the surface of individual nanotubes.
Figure 5 shows TEM images of nanotubes without and after modification. The untreated MWCNTs are cylindrical in shape and contain a hollow part (cavity) in the middle. The outer diameter of the three MWCNTs presented is 12, 13, and 16 nm, and the total number of layers observed is 12, 12, and 15 layers, respectively. The cavity diameter is approximately 5.4–6.4 nm depending on the MWCNT structure. The inset (enlarged area shown in Figure 5a) clearly shows that the untreated MWCNTs have highly ordered layers with an interplanar spacing of 0.34 nm, characteristic of carbon nanotubes [53]. However, on individual MWCNTs, the presence of the first or second layer of the outer layer with signs of disorder is observed. This is also observed for the last inner layers near the MWCNT cavity. In the TEM image of irradiated MWCNTs (Figure 5b), the outer diameter of the MWCNT ranges from 9 to 24 nm, and the inner diameter of the cavity lies between 2.1 and 5.4 nm. It can be seen that thicker MWCNTs are less damaged, while thinner ones are much more damaged. The inset in Figure 5b shows that for thick MWCNTs, damages are observed mainly in the first near-surface layers. It is clear that the interplanar distance in the near-surface region increases from approximately 0.34 to 0.35 nm. In the case of thin MWCNTs, due to the small number of internal layers, the changes are more significant. In general, we can conclude that the internal graphene structure of MWCNTs is preserved after irradiation, although some distortions are observed.
Figure 6 presents the X-ray diffraction patterns of the examined samples. Evidently, for MWCNTs, pronounced peaks manifest at 2θ = 25.7° and 43.1°, 44.6°, corresponding to reflections from the (002) and (100), (101) graphene planes, respectively [10]. On the other hand, in the case of N-MWCNTs, a general decrease in the intensity of the (002) and (100), (101) peaks is noticeable, accompanied by a shift in the most prominent (002) reflection toward smaller angles by 0.3°. These changes occur as a result of exposure to the ion beam, which causes the formation of point and extended structural defects inside nanotube walls due to the incorporation of nitrogen ions. The interplanar distances before and after irradiation were determined using the Bragg’s formula (d(002) = λ/2 sin(Ɵ), where d(002) is the interplanar spacing, λ is the wavelength of Cu Kα radiation (0.15406 nm), and Ɵ is the Bragg angle). The d(002) parameter was found to increase from 3.46 Å (untreated MWCNTs) to 3.51 Å (N-MWCNTs), which correlated with the TEM data (Figure 5). It is important to note that the (002) peak in the X-ray diffraction spectrum of irradiated MWCNTs remains, although it becomes wider and less intense. Combining these analyzed data and TEM data, we can conclude that, in general, the nanotubes retain their crystalline structure after irradiation, and the greatest damages occur mainly in the near-surface region of the MWCNT.
The evaluation of the chemical composition and charge state of atoms within the nanotubes, as well as the determination of the configurations of nitrogen inclusions formed subsequent to irradiation with N+ ions, was carried out using XPS techniques. Examination of the survey PE spectra (Figure 7) obtained in the binding energy range of 1300 to −5 eV for MWCNTs revealed the presence of only the following elements: oxygen (O1s PE line ~532 eV) and carbon (C1s PE line ~285 eV). In the case of N-MWCNTs, the N1s PE line (~400 eV) is additionally observed in the survey spectrum. Thus, we can conclude that after irradiation with a beam of nitrogen ions, there are no extraneous inclusions in the sample. For the quantification of element concentrations, the elemental sensitivity factors method [54] was employed. As depicted in Table 1, the total nitrogen concentration (Ntot) within the irradiated sample is approximately ~1.3 at.%. In addition, an increase in oxygen concentration by approximately ~5 at.% is observed.
The study of the chemical state of carbon atoms was carried out using the C1s PE spectra presented in Figure 8. The C1s PE spectrum of the pristine MWCNTs (Figure 8, bottom panel) is well fitted by six components. This is evidenced by the complete coincidence of the envelope curve, designated in brown, with the experimental signal (black curve). The C=C component at an energy of 284.7 eV (red curve) corresponds to sp2 hybridized carbon atoms in the walls of MWCNTs. The next C-C component at an energy of 285.2 eV (green curve) is associated with structural defects (mainly topologic pentagon–heptagon (5–7) Stow–Wales defects and point: mono- and di-vacancy), as well as the presence of amorphous carbon on the surface or near-surface region of MWCNTs, which are formed during the synthesis of nanotubes [13,14,55,56]. The remaining components at 286.3, 287.4, and 288.7 eV correspond to carbon atoms in the hydroxyl (C-OH), carbonyl (C=O), and carboxyl (COOH) oxygen-containing functional groups (OCFG) present on the surface of MWCNTs near structural defects [13,14,55,56]. The last component at 291 eV corresponds to the π–π* shake-up satellite, which arises due to the transition of an electron from the occupied state of the valence band to the unoccupied state of the conduction band during the C1s photoionization process [14]. This peak is observed in aromatic systems due to the presence of π-conjugated delocalized electrons and is often used to assess the degree of graphitization of a carbon material: the more intense this peak, the more sp2 bonds there are in the material [47]. Taking into account the fitting results (a low value of the FWHM parameter C1s line equal to 0.77 eV, a high-intensity C=C component, and the presence of a π–π* shake-up satellite), we can conclude that the original MWCNTs have a high degree of graphitization. This result is consistent with TEM and XRD data. It can be seen that in the C1s spectrum of irradiated N-MWCNTs (Figure 8, top panel), the main PE line is shifted toward higher binding energies by approximately ~0.2 eV relative to its position in the spectrum of MWCNTs (284.7 eV), as well as being broadened from 0.77 to 1.21 eV. This result is mainly explained by an increase in the spectrum component at 285.2 eV, which is associated with the ion-induced formation of point and extended structural defects, including the incorporation of nitrogen atoms in various configurations into the graphene walls in the near-surface region of MWCNTs. In this case, the C1s binding energy of carbon atoms linked to nitrogen atoms (C-N) according to literature data lies between 285.5 and 285.9 eV, which overlaps with the range of C1s binding energies for carbon bonds in C-C (285.1–285.6 eV) [14,57,58,59]. Thus, the component at 285.2 eV is a combination of C1s PE signals from C-C and C-N bonds. At the same time, it is clear that in the C1s PE spectrum of N-MWCNTs, the sufficiently intense sp2 component (C=C) and a π–π* shake-up satellite is observed. Considering that the exit depth of the C1s photoelectrons when using Al Kα radiation (hν = 1487 eV) is up to 2 nm, we can assume that all damages caused by ion irradiation and observed in the C1s PE spectrum occur to a large extent in the near-surface region (up to 6–7 graphene layers) of MWCNTs. In addition, it should be noted that during ion irradiation, the number of oxygen-containing functional groups (OCFGs) on the surface of MWCNTs increases noticeably (Table 1), since the intensity of their C1s PE lines becomes greater. This phenomenon is associated with the attachment of additional OCFGs at sites of structural defects formed after exposure of the irradiated samples to the atmosphere (environment) [48].
The evaluation of the chemical state of nitrogen was performed using N1s PE spectra (depicted in Figure 9a). As can be seen, in the spectrum of the pristine MWCNTs, nitrogen is completely absent, while in the spectrum of irradiated N-MWCNTs, due to fitting, three components are observed with maxima positioned at binding energies of approximately ~401.2 eV (N-Q), 399.8 eV (N-pyrr), and 398.7 eV (N-pyr). These binding energies correspond to 1s electron energies of the nitrogen atom in graphitic/quaternary, pyrrolic, and pyridinic forms [14] (as depicted in Figure 9b). It is noteworthy that the N-pyrr component has the highest intensity, and its relative contribution to the total intensity of the N1s PE line is 63%, wherein the relative proportions of N-pyr and N-Q are approximately the same and amount to 18.5%. Summing up the results of the analysis of the N1s PE spectrum of N-MWCNTs, it is important to note that as a result of irradiation with N+ ions, all nitrogen atoms are incorporated into the structure of the nanotube walls and chemically bonds with carbon in three configurations. In this case, the fraction of catalytically active nitrogen (nitrogen in pyridinic and pyrrolic configurations) accounts for about 1 at.% of the total nitrogen concentration Ntot (see Table 1).

4. Conclusions

Throughout the undertaken research, a novel approach has been devised for the incorporation of nitrogen ions within the walls of nanotubes, which have been deposited onto the surface of a titanium substrate via the spray method. It has been shown that about 1 mg of N-MWCNTs can be obtained from an area of 30 cm2 in one irradiation cycle, and up to hundreds of milligrams is possible for larger substrates. The outcomes of this study elucidate that, consequent to ion bombardment, all nitrogen is embedded into the nanotube wall structure, primarily adopting the configuration of pyridinic and pyrrolic defects (constituting approximately 80% of the total nitrogen concentration). These specific defects hold significant promise for catalytic applications. Furthermore, it has been ascertained that, beyond the catalytically active nitrogen defects, the irradiated nanotubes are enriched with oxygen-functional groups (hydroxyl and carbonyl). This supplementary feature has the potential to further amplify the catalytic activity of the material. The evolution of this nanotube modification technique stands poised to enable the manipulation of the nitrogen concentration within nanotube walls within desired thresholds, while also elevating the yield of nitrogen-infused nanotubes that exhibit the desired physicochemical attributes. These advancements are anticipated to contribute to the creation of novel materials for applications in cathodic oxygen reduction reactions.

Author Contributions

Conceptualization, P.K. and E.A.; methodology, P.K., E.A. and E.K.; formal analysis, E.K. and K.K.; investigation, P.K., K.K. and E.K.; writing—original draft preparation, P.K.; writing—review and editing, A.V., O.L., E.A. and P.K.; visualization, P.K.; supervision, E.A.; funding acquisition, E.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Russian Science Foundation, grant number 22-13-00035.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The work was performed using the equipment of the St. Petersburg State University Research Park: Centre for Physical Methods of Surface Investigation, Centre for X-ray Diffraction Studies, and Interdisciplinary Resource Centre for Nanotechnology. The authors are grateful to S.N. Povoroznyuk for help in selecting parameters and performing irradiation of CNTs.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Types of nitrogen inclusions in the nanotube wall: pyridinic (N-pyr), pyrrolic (N-pyrr), graphitic/quaternary (N-Q) forms.
Figure 1. Types of nitrogen inclusions in the nanotube wall: pyridinic (N-pyr), pyrrolic (N-pyrr), graphitic/quaternary (N-Q) forms.
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Figure 2. Scheme of preparing N-MWCNTs using nitrogen ion irradiation. The arrows show the individual stages from the beginning to the end of obtaining modified nanotubes.
Figure 2. Scheme of preparing N-MWCNTs using nitrogen ion irradiation. The arrows show the individual stages from the beginning to the end of obtaining modified nanotubes.
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Figure 3. (a) Photograph of an MWCNT layer deposited via aerosol spraying on a Ti substrate; (b) SEM image of cross-section of an MWCNT layer on a Ti substrate; (c) general view of the ion implanter; (d) ion source scheme: 1—magnets, 2—cathode, 3—anode, 4—gas inlet, 5—sample table, 6—sample; (e) calculation of the projective range of N+ ions in a layer of nanotubes 650 nm thick on a Ti substrate.
Figure 3. (a) Photograph of an MWCNT layer deposited via aerosol spraying on a Ti substrate; (b) SEM image of cross-section of an MWCNT layer on a Ti substrate; (c) general view of the ion implanter; (d) ion source scheme: 1—magnets, 2—cathode, 3—anode, 4—gas inlet, 5—sample table, 6—sample; (e) calculation of the projective range of N+ ions in a layer of nanotubes 650 nm thick on a Ti substrate.
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Figure 4. SEM images of nanotubes before and after irradiation with nitrogen.
Figure 4. SEM images of nanotubes before and after irradiation with nitrogen.
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Figure 5. TEM images of nanotubes before (a) and after (b) irradiation with nitrogen. The insets show HRTEM images of a selected section of MWCNTs.
Figure 5. TEM images of nanotubes before (a) and after (b) irradiation with nitrogen. The insets show HRTEM images of a selected section of MWCNTs.
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Figure 6. XRD patterns for nanotubes before and after irradiation with nitrogen ions.
Figure 6. XRD patterns for nanotubes before and after irradiation with nitrogen ions.
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Figure 7. Survey PE spectra of carbon nanotubes before and after irradiation with nitrogen ions (hν = 1487 eV).
Figure 7. Survey PE spectra of carbon nanotubes before and after irradiation with nitrogen ions (hν = 1487 eV).
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Figure 8. C1s PE spectra for before and after irradiation with nitrogen ions.
Figure 8. C1s PE spectra for before and after irradiation with nitrogen ions.
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Figure 9. (a) N1s PE spectra for before and after irradiation with nitrogen ions. (b) Chemical structures of possible types of nitrogen inclusions in MWCNT walls.
Figure 9. (a) N1s PE spectra for before and after irradiation with nitrogen ions. (b) Chemical structures of possible types of nitrogen inclusions in MWCNT walls.
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Table 1. Concentration of elements in at.% by XPS.
Table 1. Concentration of elements in at.% by XPS.
Sample[C][N][O]
NtotN-QN-pyrrN-pyr
MWCNTs99.16----0.84
N-MWCNTs92.981.330.270.810.255.70
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Korusenko, P.; Kharisova, K.; Knyazev, E.; Levin, O.; Vinogradov, A.; Alekseeva, E. Surface Engineering of Multi-Walled Carbon Nanotubes via Ion-Beam Doping: Pyridinic and Pyrrolic Nitrogen Defect Formation. Appl. Sci. 2023, 13, 11057. https://doi.org/10.3390/app131911057

AMA Style

Korusenko P, Kharisova K, Knyazev E, Levin O, Vinogradov A, Alekseeva E. Surface Engineering of Multi-Walled Carbon Nanotubes via Ion-Beam Doping: Pyridinic and Pyrrolic Nitrogen Defect Formation. Applied Sciences. 2023; 13(19):11057. https://doi.org/10.3390/app131911057

Chicago/Turabian Style

Korusenko, Petr, Ksenia Kharisova, Egor Knyazev, Oleg Levin, Alexander Vinogradov, and Elena Alekseeva. 2023. "Surface Engineering of Multi-Walled Carbon Nanotubes via Ion-Beam Doping: Pyridinic and Pyrrolic Nitrogen Defect Formation" Applied Sciences 13, no. 19: 11057. https://doi.org/10.3390/app131911057

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